A number of slow or persistent viral infections have been identified in animals, and several such as kuru, Jakob-Creutzfeldt disease, and subacute sclerosing panencephalitis are known in man. A number of other human chronic diseases are suspected of having a slow virus etiology. We are studying three well characterized slow virus infections in experimental animals in an attempt to define the viral agents, the host immunologic responses, and how these interact to produce disease with the hope that the information obtained will be applicable to the study of human disease. The infections being studied are Aleutian disease of mink, lactic dehydrogenase virus infection of mice, and scrapie infection of mice. Aleutian disease virus has been isolated in cell culture, and we are in the process of characterizing this virus.